The Oil & Gas industry has always considered hydrocarbons as being biogenic (i.e., produced by living organisms) and it is certain that some proportion of the hydrocarbons recovered from wells is biogenic.
However, the idea that hydrocarbons can be formed in the core of the earth and migrate through the mantle has been gaining a resurgence in acceptance. This abiotic (i.e., not derived from organisms) gas and oil would provide some unknown quantity of supplemental hydrocarbons that would have a potential for recovery and commercialization.
Exactly what that quantity would be requires some in-depth analysis.
Recoverable Hydrocarbons Explained
Recoverable hydrocarbons must first be hydrocarbons, then they must be located in a rock stratum that allows them to be stored. Finally, there must be some sort of containment to keep them from leaking out of the storage strata.
Occasionally, the cap rock has an effective seal on the reservoir. When the Oil & Gas industry comes along and drills a well and stimulates the reservoir, the flow path becomes a commercial transaction—this is the reason that the Oil & Gas industry exists.
Biotic Hydrocarbons
The total amount of carbon that is tied up within living biological material on the earth is about 4 trillion metric tons (tonne, 4.4 trillion tons). Something like 105 billion metric tons of carbon are discarded by their living host each year—the leaves of deciduous trees fall to the ground each autumn, some number of animals, plants, bacteria and viruses die each year, all organisms emit some amount of solid, liquid and gaseous waste, and so on.
This carbon will eventually become CO2 and water vapor in an aerobic environment and mostly CH4 in an anaerobic environment. The discarded biomass is approximately equally divided between land and sea.
Aerobic decomposition is exothermic and tends to be fairly rapid (i.e., from the onset of decomposition to a sterile, carbon-free mass takes weeks or months) and largely free of the worst odors. Aerobic decomposition converts the carbon in the waste material to CO2 and H2O. Anaerobic decomposition is endothermic, much slower and very smelly. It can take hundreds or thousands of years for an undisturbed anaerobic process to run to completion. The time required is largely a function of the energy input to the process. Anaerobic decomposition converts the carbon in the waste material to CH4 and amounts of CO2, and H2O limited by available oxygen.
It is common for decomposing organic material to accumulate on the sea floor away from thermal vents and begin the decomposition process. While the mass is decomposing it sometimes happens that a storm event or a seismic event will cover the biomass with sand. Above the sand over many years, more organic material will collect that can eventually turn into shale which is one of the most common cap rocks.
There is no competent theory to allow prediction of the proportion of biomass that will be subject to anaerobic decomposition. We can make some (arbitrary, but conservative) assumptions about the proportions:
- At least 1% of the biomass on land undergoes anaerobic decomposition (this includes human sanitary landfills, lakes and wetlands, and in the stomachs of many animals and insects) . 1% of half of 105 billion tonnes per year is 0.525 billion tonnes of carbon per year subjected to anaerobic decomposition.
- 50% of the biomass in the sea is subjected to anaerobic decomposition. Half of half of 105 billion tonnes per year is 26.25 billion tonnes per year.
- With these assumptions, 26.775 billion tonnes of carbon converted to methane each year through anaerobic decomposition. Adding 4 molecules of hydrogen to every molecule of carbon gives us 35.7 billion tonnes of methane per year.
- The density of methane is 0.042 lbm/SCF, so 35.7 billion tonnes per year is 5 TSCF/day
- World methane production in 2014 was 0.332 TSCF/day
Virtually all of this biotic methane will escape to the atmosphere, but some will be trapped by sediments. Again, there is no competent theory to allow a reasonably accurate method of determining the mix of trapped and escaped methane, it is a matter for conjecture, but it is unlikely to be as much as 0.1 percent. A more conservative number might be 0.005 percent.
That would mean that something on the order of 2.5 billion SCF per day (72 million SCm per day) would eventually be captured for future recovery. If this goes on for 360 million years, then the upper limit on the mass of hydrocarbons from biological sources stored in natural reservoirs is on the order of 6.9 Zettagrams (6.9 x 1021 grams).
The US Geological Survey (USGS) estimates that as of the end of 2012, cumulative worldwide production has been 1,218 billion barrels of crude and condensate and 2,438 trillion SCF of natural gas. The same source shows proved reserves of oil and gas to be on the order of 8 petagrams (8 x 1015 grams).
This indicates that we have found, developed, and/or produced 0.002 percent of the upper limit of biotic hydrocarbons.
These numbers seem to refute the idea that we have produced so much fossil fuel that it has to have another source. The five one-thousandths of a percent captured is a guess without much basis, but is unlikely to be as far off as the duration of 360 million years.
Some estimates have been made that assume seeps started within 100 years of the first organic waste being generated, so the duration of the seeps would be closer to 2 billion years. In other words, the conventional wisdom placing the source of all produced hydrocarbons as anaerobic decomposition remains plausible.
Abiotic hydrocarbons
The concept that petroleum and natural gas are formed by inorganic means has been proposed many times. One of the major theories was put forth by Thomas Gold in 1955 and expanded upon in the 1980s and 1990s. His theory was that elemental carbon and elemental hydrogen in the core of the earth at very high pressure and temperature would be adequate to facilitate the formation of hydrocarbons.
Simple hydrostatic pressure at 75 miles (120.7 km) below sea level would be about 170,000 psi (1200 MPa) and the temperature of the earth’s core is estimated to be about 10,800°F (6,000 C).
Although this theory and the competing theory that hydrocarbons arrived as space debris have both been discredited by modern science, we’ve seen this sort of group-think in other fields of inquiry recently and the collective opinion of “modern scientists” carries less weight every year.
A recent study by The Carnegie Institution’s Geophysical Laboratory was able to form ethane propane, butane, molecular hydrogen, and graphite (carbon) from methane at high temperature and pressure without any biological agents. They further found that the process was reversible, where ethane formed methane at the pressure and temperature of their experiment, which was an unexpected result.
Do Abiotic Hydrocarbons Matter to the Oil & Gas Industry?
When extracting fossil fuels, we have no way to determine if they are abiotic or biotic in source. Nor do we have any economic reason to care. Abiotic hydrocarbons are certainly not required to support the volumes of petroleum products that have been recovered to date, but recent studies indicate that the scientific community could easily have been premature in rejecting the very concept of abiotic hydrocarbons.
At the end of the day, it really does not matter if the gas and oil that arrives at a processing facility came from “squashed dinosaurs” or from chemical reactions within the core of the earth—they still had to migrate from a “source” to a “reservoir” that is “capped”.
With the specificity of the required storage environment nearly all abiotic hydrocarbons would have migrated to the surface without being trapped over geologic time just like biotic hydrocarbons did/do.